Purification of crude sulfur hexafluoride



iatented F eb. 22 .1949

rusrrroA'rroN or CRUDE suLFUa HEXAFLUQRIBE John n, ani-nth, .ra, Pennsylvania 'jSalt' Mann Philadelphia, Pa -a corp vania No Drawing. Application iuh 28 1947, Serial N0. 757,917

(cues- 2 05) 8 Claims.

. 1 This invention relates to purification of crude sulfur hexafluoride.

The known procedures for making sulfur hexafluoride (formed by burning sulfur in a fluorine atmosphere) produce a gaseous reaction product which in addition to the desired sulfur hexafluoride contains such impurities as unreacted fluorine, hydrogen fluoride, unreacted sulfur, S2F1c and lower fluorides 'of sulfur. It is known that certain of these impurities, for example, the unreacted fluorine, the hydrogen fluoride and some oithe lower fluorides of sulfur, may beremoved" by washing the crude sulfur hexafluoride reaction product with an aqueous liquid whichis preferably alkaline. some of the lower fluorides of sulfur and particularly the SzFro are not satisfactorily removed by such a wash.

I have now discovered a procedure whereby the above-named impurities may be removed from sulfur hexafluoride substantially completely in a simple and 'efficientsequence of operations. The purification process of my invention involves treating sulfur hexafluoride' which contains as impurities acid reacting gases'such as fluorine, hydrogen fluoride or both, and also S2F1o and possible other'lower fluorides of sulfur, to at least three successive treatments. Inthe flrststage of my purification process the crude sulfur hexafluoride is" washed with an aqueous.

liquid which preferably-contains in solution'an alkali such as KOI-I, NaOH; or the like, to remove the water-soluble impurities; particularly those 'of acid reaction. In'the second 'stage of my purification process the washed crude sulfur aqueous liquid, preferably an alkaline aqueous liquid, both before and after thegas is subjected to a temperature of at least 250? C., is peculiarly effective to remove the impurities present'in crude sulfur hexafluorides substantially completely. H

'In a preferred method of carrying" out; the

However,

2 ti-bess or my invention; a bride; sulfur ssa fludiide'tizisjp my cop endif gfapplication Serial "No,

. r 7 the "sar'ne day as this fapplication, entitled Productionofsulfurhexafluoridefispassediiiitofthe first" washing tower which "may be constructed for example of copper,*nickelFMoneL aH any material resistant ito the c memesof aque'ousfluorides;acidsan I The tower acked" with afmateri'al of s resistance. 7 V g y H g V The-wasnuquor in this tower maybeeithe1' water or'an aqueous solution of an alkali'sjuch o a s m h xi e 1 Sd 'in'm jhr if x Sodium carbonate, "Calcium hydroxide "OI" th like, nq n r ns .91 a a ran in 7 m amen 0L'to"50% rby"-weight are donvenient. Apreferred wash liquor isja 30 to 35%;by weight fisolutionpf potassium "hydroxide, "This readily 'ie hoves water soluble impurities from -the '"crude gas, particularly acid reacting water-solubledriipiirb ties such as any unreacted" fliiorinethat mafibe present; hydrogen' fiuoride;and lowerffiu rides of sulfur 'such ras SE4; 1 ";SzFgfif thesere present. Entraihfed'unreacted" sulfur vapor or particles; if", present, are also r'ernoved in this The gas isthen" passed through a decomposing chamber which is suppliedrwithufiicient' heat to brin'g'the'gasfto a emteramretr=25om 400C. preferably 2'75 to"'325 C.f Such temperaturesfare effective to decompose the partieular impurity SFm into SF6 and mwer suir ur florides such as SP4. This chamber may -be; constructed of copper, Monel alloys or nickel'and-may be "packed with, materials formed from these metals; or withca'rb'on or bauxite I prefer to'je 'nploy for this decomposerchamber av coppertube packed withjlumps of carbon, q

' The gas leaving the decomposer chamber; is then passed on to a secondfs'fcrubber." Ifdesired, the gas may first be co ed somewhat," for example by a cooling "jacket surrounding the conduit car'rying the' gas. "'lll e second 1 scrubber cept possibly atmospheric gases from a'ci second ,iscrubberl 1 foitmedby dcomposition of'th tiallycornpletely removed from .th

The resulting gasetmans-no-isipunti v water or may be dried and is then ready for packaging and use. Concentrated sulfuric acid,

7 refrigeration or other conventional means may be'used to dry the sulfur hexafiuoride, which is aninert gas. The gas may be then condensed directly as a solid or a liquid into pressure cylinders depending upon the temperature and pressure employed. 3

The following example is illustrative of the process of my invention.

Sulfur was burned in a fluorine atmosphere as' described in my copending application above referred 'to. The resulting gaseous reaction product continuously withdrawn contained, for a given unit of time, 2.44 pounds of sulfur hexafluoride, 1.3 pounds of hydrogen fluoride, .05 pound of SzFm, .40 pound of S'zFz and other lower fluorides of sulfur, and no unreacted fluorine. This gas was passed, to the first, scrubber in which it was washed with a 30% aqueous solution of potassium hydroxide. The scrubber was con- V 'structed of copper pipe packed with copper turnings over which the potassium hydroxide solution was trickled. Rates of flow were such as to employ 1.6 pounds ofpotassium hydroxide for the quantities of gas above given.

The gas withdrawn from the first scrubber during the reference unit of time contained 2.44

pounds of sulfurchexafluoride, .05'pound of S 2F1o and .03 pound of water. This gas was passed to adecomposer consisting of a copper pipe packed with carbon lumps and provided with an electrical heating coil to maintain a temperature of 300 C. 1 r

V -The gas effluent from'the decomposer, during the unit of time referred to, contained 2.48

' pounds of sulfur hexafluoride, .02 pound of sulfur tetra-fluoride and .03 pound of water. This it through an air-cooled coil. 7 r

. The gas thus treated was again contacted with 2.30% potassium hydroxide solution in a second scrubber of the same construction as the first l. A process for purification of crude sulfur hexafluoride containing as impurities at least one water-soluble acid reacting constituent and the lower fluoride of sulfur SZFlO, which process com- 7 prises first washing the crude gas with an aqueous wash liquor, thereafter heating the gas thus washed to a temperature above about 250 C., and thereafter again washing the gas with an aqueous wash liquor. 2. A process for purification of crude sulfur hexafluoride containing as impurities at least one water-soluble acid reacting constituent and the lower fluoride of sulfur S2F1o, which process comprises first washing the crude gas with an aqueous wash liquor, thereafter heating the gas thus washed to a temperature within the range of about 250 to 400 C., and thereafter again washing the gas with an aqueous wash liquor.

3. A process for purification of crude sulfur hexafiuoride containing as impurities at least one water-soluble acid reacting constituent and V hexafluoride containing as impurities at least efliuent gas was cooled to about 30 C. by passing one water-soluble acid reacting constituent and the lower fluoride of sulfur S2F1o, which process comprises first washing the crude gas with an aqueous wash liquor containing from 0 to of a strong alkali, thereafter heating the gas thus washed to a temperature within the range of about 275 to 325 C., and thereafter again washing the gas with an aqueous wash liquor containing from 0 to 50% of strong alkali.

5. A process for purification of crude sulfur hexafluoride containing as impurities watersoluble acid reacting constituents and lower fluoscrubber. In this scrubber .06 pound of potas-' sium hydroxide were employed for theamounts ofgas above indicated. This scrubber removed the. sulfur tetrafluoride.

The gas effluent from the second scrubber, during the unit of time, contained 2.48 pounds of V sulfur hexafiuoride and .93 pound of water. 7 This gas was passed to an accumulator where it was temporarily stored over water. V

The wet sulfur hexafluoride gas was withdrawn 7 from this storageas required for use orpackaging. The wet gas was first passed through a third scrubber constructed of steel or of lead V 7/ packed with material inert to sulfuric acid; In this-scrubber the gas was washed with 96 to 08% sulfuric acid to remove most of the water. The

Q The dried pure sulfur hexafiuoride thus produced maybe ;compressed to 200 pounds per, 'square inch, chilled to a temperature of 0 C.

and run" as a liquid into pressure cylinders. Since many modifications are possible in the lle m gas was, then subjected to intensive drying by 1 passing it through a bed of activated alumina.

rides of sulfur including S2F1o, which process comprises first washing the crude gas with an aqueous wash liquor containing from 0 to 50% of a strong alkalfthereafter heating the gas thus washed to a temperature within the range of about 275 to 325 C., thereafter again washing the gas with an aqueous wash liquor containing from 0 to 50% of strong alkali, and drying the gas from the last wash step.

'6. A process for purification of crude sulfur hexafluoride; containing as impurities at least one water-soluble acid 'reacti'ngconstituent and lower fluorides of sulfur including S2F1o, which process comprises first washing the crude gas with an aqueous wash liquor containing from about 30 to 35% by weight of KOH, thereafter heating the gas thus washed to a temperature within the range of about 275 to 325 0., thereafter again'washing the gas with an aqueous wash liquor containing from about 30 to 35% by weight of KOH, and drying the gas from the last wash step.

'7. A process for purification of crude sulfurhexafluoride containing as impurities at least one water-soluble'acid reacting constituent and the lower fluoride of sulfur S2F1o, which process comprises first washing the crude gas with an aqueous wash liquor, thereafter heating the gas to a relatively high temperature so as to decompose said SzFm, and thereafter again washing the gas with an aqueous wash liquor.

8. A process for purification of crude sulfur moisture from the gas after said second washing' by a preliminary drying step and thereafter passing the gas over activated alumina to remove substantially all remaining moisture therefrom.

JOHN F. GALL.

No references cited. 

